It could be an original character created specifically for the fanfiction. This new character could be an outcast in the Naruto world because of their bloodline, and the story could revolve around their journey to find a place where they are accepted.
Another option is that it could be a completely new version of an existing main character. For example, a Naruto with a cannibal bloodline. This would completely change his character dynamics, as he would have to struggle with this dark aspect of himself while still trying to achieve his goals of becoming Hokage and protecting his friends. It would create a very interesting 'what - if' scenario in the Naruto universe.
Another option could be a relative of an existing character. Maybe a long - lost cousin of Sasuke who has the Bakuton bloodline. This would add an interesting dynamic to the existing relationships in the Naruto story. They could initially be on a different path, but through the power of the Bakuton bloodline, their path crosses with the main characters.
An orphan with no known family could be the main character. They discover they have a hidden bloodline through a series of events. It could be a bloodline that gives them the ability to see the past or future in dreams. This character would then be on a journey to figure out where they came from and what their power means, all while trying to fit into the ninja society.
Sasuke could also be a great main character for this kind of fanfiction. With his already complex character arc in the original series, adding the kami bloodline to his story could take it in new and exciting directions. He could struggle with the power of the bloodline, perhaps fearing it would make him too much like those he has fought against in the past, or he could use it to seek revenge in a more powerful way than before.
It could be Sasuke. He already has a thirst for power and a complex backstory. Discovering an Espada bloodline would add another layer to his character. He could use the new powers to seek revenge in a more powerful way or perhaps find a new purpose in protecting the village instead of just avenging his clan.
Maybe an existing minor character from the Naruto series who is revealed to have the insect bloodline. For instance, that one student in Naruto's class who was always quiet and unassuming. As the story unfolds, we find out they have this amazing bloodline ability with insects and they start to develop it and become a key figure in the village's defense.
I think Naruto himself would be an amazing character with the gravity bloodline. Imagine him combining his existing Rasengan with the power of gravity. He could create a Rasengan that not only spins but also has a crushing gravitational force. Also, Naruto's never - give - up attitude would be perfect for mastering the complex abilities that come with this bloodline. He would constantly push himself to find new ways to use it in battle and for the good of the village.
It could be a new character created specifically for the fanfic. Maybe an orphan who discovers they have this unique portal bloodline and has to navigate the Naruto world while hiding their power from those who might want to use it for evil.
Naruto himself could be the main character with a devil bloodline twist. Maybe he was born with it but it was hidden until a certain event triggered it. This would completely change his journey. Instead of just trying to gain the respect of the village as a Jinchuriki, he would also have to deal with the implications of having a devil bloodline, which might be seen as even more dangerous by some in the village.
Sasuke could be an interesting choice. He already has a complex personality and adding the mind control bloodline to his abilities would make his character even more multi - faceted. He could use it to further his revenge plans or, in a different turn, to protect his friends and the village.
Perhaps a dark entity from another dimension. This entity senses Naruto's powerful bloodline and wants to possess him or use his body as a vessel to enter the Naruto's world and wreak havoc. It would have otherworldly powers that Naruto has never faced before, making it a truly formidable antagonist.